The presence or absence of P3 and the projection of the anterior root of P4 in relation to the masseteric knob.—Only rarely is P3 absent in Eutamias or present in Tamias. P3 in specimens of old adult Eutamias, shows wear, thus suggesting that P3 is functional in older chipmunks. In Eutamias, which normally has a P3, the anterior root of P4 projects to the outside of the masseteric knob, whereas in Tamias, which normally lacks a P3, the anterior root of P4 projects directly to the masseteric knob or to the lingual side of this structure. The projection of the anterior root of P4 seems to be correlated with the presence or absence of P3. However, in a specimen of Tamias striatus rufescens (No. 11117 KU), the left P3 is present, yet the anterior root of P4 still projects to the lingual side of the masseteric knob.
Ellerman (1940:48) and Bryant (1945:368-369, 372) think that the presence or absence of P3 is not of generic significance in chipmunks, since P3 is vestigial and probably is in the process of being lost, and since this character is rarely used as a generic character in other sciurids. I think that the presence or absence of P3, together with the projection of the anterior root of P4 in relation to the masseteric knob, is of generic significance, for, squirrels in general have retained the dentition and dental formula of a primitive rodent, and any change in the pattern of the teeth or in dental formula is, in my opinion, of a fundamental nature.
Length of tail in relation to total length.—The tail in Eutamias is more than 40 per cent of the total length, whereas in Tamias the tail is less than 38 per cent of the total length. In this respect Tamias resembles most ground squirrels of the genus Spermophilus.
Color pattern.—The chipmunks vary but little in color pattern, for, even in Eutamias dorsalis, which is one of the most aberrant of the chipmunks in color pattern, the pattern is characteristic of Eutamias.
The width of the longitudinal stripes is uniform in Eutamias whereas in Tamias the dorsal, longitudinal light stripes are more than twice as wide as the other stripes.
In Eutamias, only the two lateralmost dark stripes are short, whereas in Tamias all four of the lateral dark stripes are short; none extends to the rump or to the shoulder.
The dark median stripe is present in both Eutamias and Tamias as well as in other genera such as Callosciurus and Menetes (Ellerman 1940:390).
Characters in which the Subgenus Eutamias and the Genus Tamias Agree,
but Differ from the Subgenus Neotamias
Shape of the infraorbital foramen.—In the subgenus Eutamias and in the genus Tamias the infraorbital foramen is rounded, whereas in most species of the subgenus Neotamias the foramen is slitlike. In Eutamias townsendii, however, the infraorbital foramen is rounded as much as in the subgenus Eutamias and in the genus Tamias.
Width of the postorbital process at base.—The postorbital process is broader at the base in the subgenus Eutamias and in the genus Tamias than in most species of the subgenus Neotamias. In E. townsendii, however, this process is relatively as broad as in the subgenus Eutamias and in the genus Tamias.